Photodiodes are used in a variety of applications for converting light into electrical signals. For example, photodiodes are employed in photo-detection applications, such as photodetectors for detecting light and solar cells for converting solar radiation into electrical energy. FIG. 1 shows one currently available design for a photodiode 10. The photodiode 10 includes a p-region 12 made from a p-type semiconductor material and an n-region 14 made from an n-type semiconductor material that, together, form a p-n junction 16. A depletion region 18 is formed in the p-region 12 and the n-region 14 by majority-carrier holes in the p-region 12 diffusing into the n-region 14 and majority-carrier electrons in the n-region 14 diffusing into the p-region 12. The diffusion of majority carriers proceeds until an equilibrium junction potential is formed across the depletion region 18 that prevents further diffusion of the majority carriers across the p-n junction 16 from either the p-region 12 or the n-region 14.
The junction potential of the depletion region 18 provides the p-n junction 16 with the familiar, nonlinear-current-voltage characteristics shown in FIG. 2 as I-V curve 20. Under a forward bias voltage (i.e., positive voltage), the junction potential and the width of the depletion region 18 is reduced. Majority-carrier electrons from the n-region 14 and majority-carrier holes from the p-region 12 have sufficient energy to overcome the junction potential and diffuse across the p-n junction 16 to generate a diffusion current. Under a reverse-bias voltage (i.e., negative voltage within the third quadrant of the graph shown in FIG. 2), the junction potential and the width of the depletion region 18 increases dramatically, preventing diffusion of majority carriers from either the p-region 12 or the n-region 14. However, a supply of minority carriers on each side of the p-n junction 16 is generated by thermal excitation of electron-hole pairs. For example, thermally generated electron-hole pairs generated near or in the depletion region 18 on the p-region 12 side of the p-n junction 16 provide minority-carrier electrons. When the electron-hole pairs are generated within a diffusion length of the depletion region 18, the minority-carrier electrons can diffuse into the depletion region 18 and the junction potential sweeps the minority-carrier electrons across the p-n junction 16. Similarly, thermally generated electron-hole pairs generated near or in the depletion region 18 on the n-region 14 side of the p-n junction 16 provide minority-carrier holes and the junction potential sweeps the minority-carrier holes across the p-n junction 16. The drift of minority carriers generates a drift or generation current that is relatively independent of the applied voltage because the minority carriers are generated by an external source, such as thermal or optical energy.
The photodiode 10 may be used as a photodetector for detecting light incident on the p-n junction 16 by exploiting the voltage independence of the generation current. As shown in FIG. 1, electron-hole pairs are generated by illuminating the p-n junction 16 and surrounding regions with light having an energy Elight greater than the energy-band gap Egap of the semiconductor material used for the p-region 14 and the n-region 16. Under a reverse-bias voltage, optically-generated-minority carriers within the depletion region 18 or within a diffusion length of the depletion region 18 are swept across the p-n junction 16 by the junction potential to generate an optical-generation current gn. As shown in FIG. 2, the greater the amount of light incident on the p-n junction 16, the greater the magnitude of the optical-generation current. For example, the magnitude of the optical-generation current g2 is greater than the magnitude of the optical-generation current g1 as a result of a higher intensity of light illuminating the p-n junction 16 and the surrounding regions. Thus, the optical-generation current generated by illuminating the p-n junction 16 with light may be used for measuring illumination levels.
An array of the photodiodes 10 may also be used to form a solar-cell array. In a solar-cell array, each of the photodiodes 10 can be operated in the fourth quadrant of the I-V curves shown in FIG. 2 and connected to a common bus that delivers power to a load.
In order to maximize the photoconductive response of the photodiode 10, it is important that optically-generated carriers have a sufficiently long diffusion length so that the optically-generated carriers do not recombine or become trapped prior to diffusing into the depletion region 18 or while being swept across the p-n junction 16. Control of the carrier diffusion length imposes challenges on designers and manufacturers of photodiodes that can necessitate using high-quality and high-cost semiconductor materials, such as single-crystal semiconductor materials. It is often desirable for the depletion region 18 to be sufficiently wide so that a large fraction of the intensity of light is absorbed within the depletion region 18. While increasing the width of the depletion region 18 can improve the optical efficiency of the photodiode 10, it can also deleteriously decrease the response time of the photodiode 10. Therefore, manufacturers and designers of photodiodes continue to seek improved photodiodes in which cheaper, lower-quality semiconductor materials can be utilized without substantially degrading photodiode performance.